Hayward H-Series AO/AC Error Codes: Blower Vacuum Switch Troubleshooting
Quick Summary
- AO code means the vacuum switch is open when it should be closed (blower not creating enough vacuum)
- AC code means the vacuum switch is closed when it should be open (stuck closed or shorted)
- Most common cause is blocked venting or failed blower motor, not the vacuum switch itself
- Always check for vent blockages and verify blower operation before replacing components
Understanding AO and AC Codes
The Universal H-Series heater uses a vacuum switch (also called air pressure switch or APS) to verify proper combustion air flow before allowing ignition. This critical safety device ensures toxic combustion gases are properly vented outside.
AO (Air Open): The vacuum switch remains open even though the blower is commanded to run. This means insufficient vacuum/draft is being created.
AC (Air Closed): The vacuum switch is closed when it should be open (blower not running). This indicates a stuck switch or electrical short.
How the Vacuum Switch System Works
The combustion air system consists of:
- Induced draft blower: Creates negative pressure to pull combustion gases through heat exchanger
- Vacuum switch: Senses the negative pressure created by the blower
- Vent system: Exhaust pipe carrying combustion products outside
- Control board: Monitors switch status and controls blower operation
When there's a call for heat, the control board starts the blower. Once adequate vacuum is sensed (switch closes), the ignition sequence begins. If the switch doesn't close within 90 seconds, you get an AO code.
Common Causes by Code Type
AO Code Causes (Open When Should Be Closed)
Venting Issues (Most Common)
- Blocked vent termination (leaves, bird nests, snow)
- Collapsed or restricted vent pipe
- Excessive vent length or too many elbows
- Wrong vent pipe size
- Missing or damaged vent screen
Blower Problems
- Failed blower motor
- Blower wheel damaged or loose
- Debris blocking blower inlet
- Blower running backward (miswired)
- Capacitor failure (if applicable)
Vacuum Switch Issues
- Clogged vacuum tube
- Cracked or disconnected tubing
- Failed vacuum switch (won't close)
- Water in vacuum tube (condensation)
- Switch out of calibration
AC Code Causes (Closed When Should Be Open)
- Vacuum switch stuck closed mechanically
- Shorted vacuum switch
- Shorted wiring in switch circuit
- Control board sending false reading
- Moisture causing electrical short
Step-by-Step Troubleshooting
Initial Observation (All Codes)
1. Note When Error Occurs
- Does error appear immediately at startup?
- Does blower attempt to run before error?
- Any unusual sounds from blower?
- Check if error is consistent or intermittent
2. Visual Inspection
- Look at vent termination for obvious blockage
- Check if blower is visibly running
- Listen for air movement in vent pipe
- Look for disconnected wires or tubing
AO Code Troubleshooting
3. Check Vent System
- Inspect vent termination for blockages
- Remove vent screen and check for debris
- Look down vent pipe with flashlight
- Verify vent pipe is properly supported (no sags)
- Check that vent length doesn't exceed manufacturer limits
4. Test Blower Operation
- Remove blower from heater
- Apply 120V directly to blower motor
- Blower should run smoothly without grinding
- Check that wheel is tight on shaft
- Verify proper rotation direction (marked on housing)
5. Inspect Vacuum Tubes
- Trace vacuum tubing from switch to blower housing
- Look for cracks, kinks, or disconnections
- Disconnect and blow through tubes to clear blockages
- Check for water in tubes (common in humid climates)
6. Vacuum Switch Testing
- Disconnect vacuum tube from switch
- Use manometer to measure actual vacuum at tube
- Should see 0.5-1.5" W.C. negative pressure with blower running
- Apply vacuum to switch manually with vacuum pump
- Switch should close at specified setpoint (usually 0.4-0.6" W.C.)
AC Code Troubleshooting
7. Test Switch in Open State
- Turn off power to heater
- Disconnect wires from vacuum switch
- Test continuity across switch terminals
- Should show open circuit (infinite resistance)
- If showing continuity, switch is stuck closed
8. Check for Shorts
- Inspect wiring from switch to control board
- Look for pinched or melted wires
- Test each wire to ground for shorts
- Check connector pins for corrosion or moisture
Advanced Diagnostics (Technician Level)
Safety Warning
These procedures involve live electrical testing and removal of safety devices. Only qualified technicians should proceed. Never operate the heater with safety switches bypassed.
9. Blower Electrical Testing
- Measure voltage at blower during call for heat
- Should see 120VAC from control board
- Check blower amp draw with clamp meter
- Compare to nameplate rating
- High amps indicate mechanical binding
10. Control Board Diagnostics
- Check for 120V output to blower on call for heat
- Verify control board is reading switch status correctly
- Look for burned components on board
- Test board ground connection
11. System Pressure Test
- Connect manometer to multiple test points
- Measure pressure drop across heat exchanger
- Check for internal blockages or collapsed baffles
- Compare readings to manufacturer specifications
Temporary Testing Procedures
Bypass Test (Diagnostic Only)
- Turn off power to heater
- Jumper vacuum switch terminals temporarily
- Power on and see if blower runs and error clears
- IMMEDIATELY remove jumper after test
- Never operate heater with switch bypassed
If error clears with jumper, problem is vacuum switch or insufficient vacuum. If error remains, problem is control board or blower circuit.
Component Replacement Procedures
Vacuum Switch Replacement
- Turn off power and gas to heater
- Photograph wire connections before removal
- Disconnect electrical connectors
- Remove vacuum tube from switch nipple
- Unscrew switch from mounting bracket
- Install new switch in reverse order
- Verify switch rating matches original
Blower Motor Replacement
- Disconnect power and remove blower assembly
- Mark blower wheel position on shaft
- Remove wheel setscrew and slide off shaft
- Unbolt motor from blower housing
- Transfer wheel to new motor at same position
- Verify rotation direction before reassembly
- Check that wheel doesn't rub on housing
Model-Specific Information
H-Series Models with Hot Surface Ignition
- Use vacuum switch part #HADXAPS1930
- Switch setpoint: -0.50" W.C. typically
- Blower pre-purge time: 15-30 seconds
Low NOx Models
- May have different vacuum switch setpoints
- Blower speeds may vary from standard models
- Check specific model documentation
Common Parts Required
- Vacuum Switch: HADXAPS1930 (most models)
- Blower Motor: HADXBLW1930 (varies by model)
- Vacuum Tubing Kit: HADXVTK1930
- Blower Wheel: HADXBWL1930
- Blower Capacitor: HADXCAP1930 (if applicable)
- Control Board: Model-specific part number
Prevention and Maintenance
- Inspect and clean vent termination seasonally
- Check vacuum tubes for cracks annually
- Keep blower inlet screen clean
- Ensure vent screen is in place to prevent pest entry
- Clear condensation from vacuum tubes in humid climates
- Lubricate blower motor bearings if serviceable
- Verify proper vent slope to prevent condensation buildup
Venting Guidelines
Maximum Vent Lengths
- Varies by model and pipe size
- Each 90° elbow equals 10 feet of straight pipe
- Each 45° elbow equals 5 feet of straight pipe
- Consult installation manual for specific limits
Proper Vent Installation
- Minimum 1/4" per foot upward slope
- Support every 5 feet horizontally
- Use approved vent material (PVC, CPVC, or stainless)
- Seal all joints properly
- Maintain clearances from combustibles
Frequently Asked Questions
Can I clean the vacuum switch instead of replacing it?
Vacuum switches are sealed units and cannot be cleaned internally. If the switch tests bad, replacement is necessary. You can clean the vacuum tube connections and ensure the sensing port is clear of debris.
Why does the AO code only appear on windy days?
Wind can create back pressure in the vent system, preventing the blower from creating adequate vacuum. Installing a wind-resistant vent cap or relocating the termination may help.
How do I know if my blower motor is running backward?
Check the rotation arrow on the blower housing. You can also feel for exhaust air at the vent termination - there should be strong airflow when the blower runs. Reversed rotation greatly reduces airflow.
Can a dirty heat exchanger cause AO codes?
Yes, a severely sooted or scaled heat exchanger can restrict airflow enough to prevent proper vacuum. This usually indicates combustion problems that need addressing beyond just clearing the code.
Is it normal for the vacuum tube to have water in it?
Small amounts of condensation can occur, especially in humid climates. However, tubes should not be full of water. Ensure proper drainage and consider adding a condensation trap if the problem persists.